From: "jianchao.wang" <jianchao.w.wang@oracle.com>
To: Jens Axboe <axboe@kernel.dk>, Ming Lei <ming.lei@redhat.com>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V7 1/4] blk-mq: refactor the code of issue request directly
Date: Thu, 15 Nov 2018 09:35:38 +0800 [thread overview]
Message-ID: <d7c48fb9-9294-49ff-bb18-67a5db3ea021@oracle.com> (raw)
In-Reply-To: <42861ece-5519-bb59-e3a7-8fea1c731f19@kernel.dk>
On 11/14/18 11:22 PM, Jens Axboe wrote:
> On 11/14/18 2:43 AM, Ming Lei wrote:
>> On Wed, Nov 14, 2018 at 05:23:48PM +0800, jianchao.wang wrote:
>>> Hi Ming
>>>
>>> On 11/14/18 5:11 PM, Ming Lei wrote:
>>>>>
>>>>> - if (!blk_mq_get_dispatch_budget(hctx))
>>>>> - goto insert;
>>>>> + if (unlikely(!blk_mq_get_dispatch_budget(hctx)))
>>>>> + goto out_unlock;
>>>> The unlikely annotation is a bit misleading, since out-of-budget can
>>>> happen frequently in case of low queue depth, and there are lots of
>>>> such examples.
>>>>
>>>
>>> This could be good for the case for no .get_budget and getting budget success.
>>> In case of out-of-budget, we insert the request which is slow path.
>>
>> In case of low queue depth, it is hard to say that 'insert request' is
>> done in slow path, cause it happens quite frequently.
>>
>> I suggest to remove these two unlikely() since modern CPU's branch prediction
>> should work well enough.
>>
>> Especially the annotation of unlikely() often means that this branch is
>> missed in most of times for all settings, and it is obviously not true
>> in this case.
>
> Agree, unlikely() should only be used for the error handling case or
> similar that does indeed almost never trigger. It should not be used
> for cases that don't trigger a lot in "most" circumstances.
>
That's really appreciated for all of your kindly response.
Fair enough with 'unlikely'.
I will remove these two wrong 'unlikely' in next version.
Thanks
Jianchao
next prev parent reply other threads:[~2018-11-15 1:35 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-14 8:45 [PATCH V7 0/4] blk-mq: refactor and fix on issue request directly Jianchao Wang
2018-11-14 8:45 ` [PATCH V7 1/4] blk-mq: refactor the code of " Jianchao Wang
2018-11-14 9:11 ` Ming Lei
2018-11-14 9:23 ` jianchao.wang
2018-11-14 9:43 ` Ming Lei
2018-11-14 15:22 ` Jens Axboe
2018-11-15 1:35 ` jianchao.wang [this message]
2018-11-14 8:45 ` [PATCH V7 2/4] blk-mq: fix issue directly case when q is stopped or quiesced Jianchao Wang
2018-11-14 9:20 ` Ming Lei
2018-11-14 9:29 ` jianchao.wang
2018-11-14 9:35 ` Ming Lei
2018-11-15 1:37 ` jianchao.wang
2018-11-14 8:45 ` [PATCH V7 3/4] blk-mq: issue directly with bypass 'false' in blk_mq_sched_insert_requests Jianchao Wang
2018-11-14 8:45 ` [PATCH V7 4/4] blk-mq: replace and kill blk_mq_request_issue_directly Jianchao Wang
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